1. Field of the Invention
This invention relates to electrical cabling and, more particularly, to a partial discharge resistant electrical cable and a method for manufacturing the cable.
2. Description of Related Art
Generally, oilfield wireline operations concern the testing and measurement of geologic formations proximate a well periodically prior to completion or after the well has been fully drilled. Electrical power requirements for tools used to test and measure the geologic formations have increased over time as the capabilities of the tools have improved. Accordingly, cables used to deliver electrical power to the tools are required to handle greater amounts of power.
As the electrical voltage applied to a cable exceeds a critical value, generally known as the inception voltage, a partial discharge of an electrical field within the cable, produced by the electrical voltage across the cable's conductor, may occur. Referring to FIG. 1, conventional cables may contain voids 102 between a conductor 104 and an insulating layer 106 surrounding the conductor 104. Partial discharge may occur within the electrical cable 100 when air or other gases trapped within the voids 102 become ionized by the electrical field. Accordingly, it is generally desirable to at least minimize air or other gases that may have entrapped between the conductor and the insulation.
Generally, conventional wireline cables include stranded copper conductors insulated with fluoropolymers or polyolefins. It is desirable for the insulating materials to be strong, wear resistant, and capable of withstanding high temperatures, so that they are able to tolerate environments typically encountered during manufacturing and use. Such polyolefin-type polymers can generally be easily compression extruded in small thicknesses onto stranded copper conductors at economically viable speeds, producing insulated conductors having substantially no air or other gases entrapped between the conductor and the insulation.
However, such fluoropolymers are generally very difficult to compression extrude through small die orifices to produce thin layers of insulation on conductors at economically viable speeds. Secondary bonding forces (such as Van der Waal's forces) within simple hydrocarbons, such as polyolefin-type polymers, may generally be about 40 KJoules/mole, while such forces within fluoropolymers may generally be about 4 KJoules/mole. Thus, fluoropolymers generally achieve their strength and toughness by having molecules with very high molecular weights that entangle with neighboring molecules to compensate for the low secondary bonding force. The high molecular weight of the fluoropolymers leads to considerably higher viscosities at their processing temperatures than other polymeric insulation materials. Further, many fluoropolymers may experience severe melt fracture, visible as excessive surface roughness, when compression extruded in small thicknesses due to their high molecular weights.
Accordingly, fluoropolymer insulation is typically extruded through large die orifices and the material is stretched, while in a melted state, to a desired thickness and shaped onto the conductor. While this process may produce cabling at economically viable speeds, air or other gases are often trapped between the conductor and the insulation.
The present invention is directed to overcoming, or at least reducing, the effects of one or more of the problems set forth above.